Rock-bolts are used in mining, tunneling, and in general stabilization. One use of rock-bolts is in the coal mining industry where underground roadways or tunnels are excavated to facilitate the main mining operation. The tunnels have to be reinforced for safety reasons. Traditionally this has been done with steel rods called rock-bolts or rock-dowels. Such reinforcement may be of a permanent or temporary nature, and where the reinforcement is of a temporary nature, the reinforced strata may be subsequently excavated/mined.
However, when a steel rock-bolt is used to reinforce this part of the strata, expensive damage can occur to the excavating equipment used in the later excavation and also to equipment used in conveying the excavated material.
Because of this problem with steel rock-bolts, fibre reinforced composites (FRP) rock-bolts have become popular, particularly for temporary applications. Such materials have lower shear strength characteristics than steel and will not tend to damage the excavating or conveying equipment. FRP rock-bolts are also used in other applications such a “soil nailing” where the bolts, when used for temporary purposes, can be easily broken or cut up and removed at a later date when or if required.
In coal mining, the use of fibre reinforced dowels or bolts is limited to the mining or “later to be excavated” side of the access tunnel (called ribs) or what is commonly called the “cuttable” side. Steel rock-dowels are usually used in the other non “cuttable” side and the roof of the access tunnel.
A typical rock bolt or dowel used in coal mines is usually a rod of 20 mm-22 mm diameter and varying length from 900 mm-1800 mm, which is inserted in a pre-drilled hole of approximately 28 mm diameter and encapsulated in a binding cementitious material, usually a resin material.
In many cases, the rod has a threaded end that projects out of the hole where a washer and nut are attached to the rod. After encapsulation, the nut is tightened down to exert a pressure on the strata surface.
The sequence of dowel installation is, firstly a hole is drilled in the strata to the required depth, the drill bit is then removed from the drill chuck and replaced by a socket spanner. A two part resin binding agent contained in a flexible capsule of varying length is inserted into the hole. The capsule keeps the two components separate. Then the dowel, including a plate and a nut partially screwed onto the threaded end is partly inserted into the hole.
The nut is engaged by the drill chuck and spun vigorously whilst being pushed further into the hole, thus breaking the capsule and mixing the two resins together. The nut has a cap which prevents it from being screwed further down the dowel thread during the spinning operation, the dowel is then held motionless for a number of seconds whilst the, now mixed, resin solidifies.
When the resin is hardened the nut is turned further down on the now rigidly held encapsulated dowel which breaks out a cap on the end of the dowel at a pre-determined torque value and allows the nut to be tightened, creating force on the washer plate and strata surface until the desired torque value is attained. This value is determined by the skill of the drill rig operator.
The torque value is usually accomplished by guesswork which can be quite difficult as working conditions are usually tricky. Mines tend to be poorly lit and the equipment is robust and very strong. Typically, the operator cannot see if he has damaged the dowel by over tightening of the nut, nor can he tell if the encapsulation is adequate or successful.
In many cases, the machines that excavate the access tunnel also install the reinforcement dowels at the same time. These machines have drilling rigs positioned on the back of the machine and the sides and the roof of the tunnel are reinforced with dowels as the tunnel forming machine advances.
The drilling rigs are operated hydraulically and are basically designed to install steel rock-dowels. Thus, when installing the FRP dowels on the “cuttable” side, a problem arises due to the high torque performance of the drill rig required for the steel dowels and the low torque values of the FRP dowels.
The strength of the installation drill rig and the significant difference in shear and torque values between steel and FRP dowels, results in the FRP dowel being easily damaged unless the operator is experienced, skilled and very careful. In extreme cases the head of the dowel is twisted off.
Some mines have “automatic bolters” mounted on the tunneling machine, which cannot be used effectively with FRP dowels because of the difference in torque values between steel and FRP dowels.
Because the FRP dowel is not visible to the operator, it can be damaged without the operator's knowledge. In the past, damaged FRP dowels have caused walls to collapse resulting in severe injuries to mining personnel. These incidents have prompted officially written safety warnings by State mining authorities concerning the use of fibreglass or composite dowels.
To overcome this problem some FRP dowel manufacturers have developed what is known as a “thrust” dowel. This type of dowel has an enlarged nut shaped head but has no thread to exert force onto the strata surface. The installation drill rig simply pushes the head of the dowel hard into the hole until the encapsulating binder solidifies.
However, when using a thrust dowel the operator cannot tell if there is sufficient load onto the strata or more importantly if the encapsulation has worked satisfactorily, which is critical for the safety of mining personnel. Hence there are safety issues with the use of thrust dowels.
One additional problem with using a threaded end FRP rock-dowel, is that when the nut is tightened, the threaded end will project into the tunnel where there is typically constant traffic. The tunnels are relatively cramped. This can result in vehicles bumping or hitting the projecting end and damaging the dowel head and shank.
Although the use of a thrust dowel overcomes the problem of vehicular damage to the projecting FRP bolt, the performance of thrust dowels is significantly inferior to a threaded dowel in that the force applied to the strata surface is less than one third to that of the threaded dowel.
In addition the use of thrust dowels does not overcome one of the main causes of strata failure in the first place, being the poor encapsulation of the dowel within the pre-drilled hole. There are a number of separate issues which may lead to encapsulation failure. Such encapsulation failures are not apparent to the dowel installation operator and therefore can lead to significant personnel safety issues.
The encapsulation failure issues include the encapsulating resin not being mixed thoroughly and therefore not developing enough strength. Another failure issue arises where the encapsulating resin is insufficient in volume to bond enough of the dowel length. This allows the strata material between the dowel head and the end of the encapsulation to fall away.
The two part resin is divided into two separate casings. If the spinning dowel does not break both casings, the resins cannot be mixed. The dowel must be designed to ensure that both casings are broken open. Failure occurs where this does not happen.
As the spinning dowel is pushed further into the hole, it must force the broken resin casings down into the bottom of the hole. If it fails to do this, the casing becomes wrapped around the spinning dowel again causing the bonding between the resin, strata and the dowel to fail. This is a very common type of failure, known in the art as “gloving”.
The failure of mining personnel to be aware of the above potential causes of failure during and after dowel installation, means that the strata may not be adequately reinforced. This is potentially dangerous and a known health hazard.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.